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Reinsurance - Narrative (Details) - USD ($)
$ in Thousands
Jul. 31, 2019
Mar. 31, 2024
Dec. 31, 2023
Mar. 31, 2023
Dec. 31, 2022
Dec. 31, 2018
Dec. 27, 2018
Reinsurance Retention Policy [Line Items]              
Reinsurance recoverable on unpaid losses (Allowance for expected credit losses: 2024 - $2,438; 2023 - $3,240)   $ 569,346 $ 564,331        
Reinsurance recoverable, allowance for credit loss   2,438 3,240 $ 4,254 $ 4,277    
Reinsurance, excess retention, amount reinsured, per policy $ 2,178,535            
Reinsurance, amount retained, per policy 600,000            
Ceded cumulative losses $ 445,000            
Reinsurance recoverable for paid and unpaid claims and claims adjustments, adverse development cover           $ 155,000  
Reinsurance recoverables, including reinsurance premium paid   569,346 564,331 $ 552,513 $ 556,116    
Deferred gain on retroactive reinsurance   78,222 73,240        
Collateral by third party   473,144 490,070        
Maximum              
Reinsurance Retention Policy [Line Items]              
Reinsurance recoverables on paid losses, gross   312,786          
Diversified Reinsurance              
Reinsurance Retention Policy [Line Items]              
Reinsurance recoverable on unpaid losses (Allowance for expected credit losses: 2024 - $2,438; 2023 - $3,240)   43,094 43,176        
Reinsurance recoverable, allowance for credit loss   2,284 2,769        
Assets and liabilities associated with reinsurance business retroceded             100.00%
AmTrust Reinsurance              
Reinsurance Retention Policy [Line Items]              
Reinsurance recoverable, allowance for credit loss   136 453        
Reinsurance recoverables, including reinsurance premium paid   520,780 515,463        
Deferred gain on retroactive reinsurance   $ 75,916 $ 70,916